The following correction was printed in the Guardian's Corrections and clarifications column, Tuesday 10 August 2010

The recent killing in Afghanistan of a group of medical workers – among them Karen Woo – was referred to in this story as an execution. The Guardian style guide says this term should not be bestowed on extrajudicial killings. "Execution: the carrying out of a death sentence by lawful authority, so a terrorist, for example, does not 'execute' someone"

Karen Woo travelled to the remote Nuristan region of northern Afghanistan with hopes of making a difference, with undimmed passion to help others, and with no little trepidation for the physical and mental challenges ahead. One thing she would not have travelled with was a Bible.

Of that her grieving family are convinced. Mourning the loss of a vivacious daughter and sister who "combined brains and beauty, intelligence, drive and kookiness in equal measure" they spoke of an exceptional young woman, a humanist whose motivation was purely humanitarian, but who had no religious or political agenda.

Yet, as further details emerged of the execution of the young British surgeon and nine others from a medical team in this beautiful but increasingly lawless mountain region, the fact she was with an overtly Christian organisation cannot be dismissed as insignificant.

Throughout Afghanistan, the International Assistance Mission (IAM), an umbrella organisation representing Christian agencies, is associated – rightly or wrongly – with claims of proselytizing.

Though long established in Afghanistan, after operating in the country since 1966, it was to find itself targeted by the Taliban regime in 2001, its workers expelled, its offices in Kabul shut. Tom Little, from New York and the team leader of this ill-fated expedition to provide eye care to the almost inaccessible villages of Nuristan, was out of the country during the time of the expulsions but returned later.

Details of what occurred after the eight foreign medical workers and their Afghan staff stopped for a picnic in a forest high in the Hindu Kush in Badakhshan province are sketchy.

Reports are of the group being ambushed by gunmen with long red-dyed beards, according to local police. They are believed to have reached their vehicles after trekking on foot and with mules from roadless Nuristan, their route chosen because it skirted areas where insurgents could be present.

The gunmen are said to have robbed the team – comprising Dr Woo, who was due to marry in London in two weeks, six Americans, a German and three, possibly four, Afghan interpreters and drivers. Their bodies were found on Friday.

The manner of their deaths raises several questions, including why it was necessary to execute them, to line them up alongside their vehicles and shoot them one by one.

In an area roamed by bandits and ruled by drugs mafia, robbery was always a risk. But the testimony of one Afghan driver named Saifullah, the only one of the group to escape, points to religion as a motive. Now in Kabul to help police with the investigation, he has told them of being marched uphill as he recited the Qur'an, praying to be spared, swearing over and over again he was a true Muslim.

One unconfirmed report said the gunmen spoke a Nuristani dialect. Badakhshan's deputy police chief, General Sayid Hussain Safari, has speculated the group may have been tracked from Nuristan before being ambushed as they reached their vehicles. Investigators cannot rule out they were targeted because they were Christians.

That the Taliban have claimed responsibility makes the picture no clearer. Taliban spokesman Zabiullah Mujahid said the aid workers had ignored orders from an insurgent patrol to stop, and were shot as they tried to flee. Among their possessions, he said, was a Bible in the local Dari language, and maps illustrating the location of the Taliban in northern Afghanistan, suggesting they were not there to deliver medication, but to proselytize and to spy.

Yesterday Dirk Frans, the executive director of IAM, said: "The accusation is completely baseless, they were not carrying any Bibles, except maybe their personal Bibles. As an organisation we are not involved in proselytising at all."

The Taliban's claim goes against their way of working which is to capture, not kill, then barter for ransom or the release of key prisoners.

Firuz Rahimi, an Afghan-born BBC World Service journalist and friend of Dr Woo, with whom he co-founded the Bridge Afghanistan organisation, travelled to the country last year to make a documentary. He said: "There are lot of indications that would say this is not the Taliban. It's a very confusing picture."

The Taliban has targeted foreign aid workers in the past but such attacks are rare and they have even granted safe passage to aid workers in some areas they control. In July 2007, they did seize 23 South Korean missionaries and two hostages were killed, but the rest were released after negotiations. IAM argues that it never attempts to hide the fact it is a Christian organisation. The organisation is generally regarded as culturally sensitive.

One NGO security consultant, who asked not to be named, said it had a reputation among other agencies for combining missionary work with aid operations. "A lot of the big Christian aid outfits are very careful to avoid doing that but IAM had a name for doing a bit of spreading the word if they had the chance," he said.

"The combination of the Christianity and the way they did security could have made them a soft target. They've been in the country so long that they are a known quantity in terms of how they operate."

Those who knew Dr Woo say her motivation was not religious. Mark Von Koeppen, a construction contractor in Kabul, said: "I can't speak on behalf of the rest of the convoy but Karen Woo was not there to convert. Her only goal was to help the people that needed help."

Rahimi agreed. "She was always looking for opportunities. My guess is the IAM team planned to go there and she thought she could be useful. It is not at all the case that she was religious. I don't know about the rest of the team but I am 100% sure about her."

She would have felt secure, he said, travelling with such an experienced group. "I spoke to her on the night she was going. She knew all about the security problems. It's not something she would have ignored. They chose a safe route. She was much more concerned about the physical demands of the walk, with the horses. And she was really excited because she was going, and she was going to able to do something".

Family's statement

How the family of Dr Woo paid tribute to her:

Our loving and talented daughter/sister Karen Su Ying (Cori) Woo has been tragically taken from us. She was due to be married on her return to the UK. Her motivation was purely humanitarian. She was a humanist and had no religious or political agenda.

She wanted the world to know there was more than a war going on in Afghanistan, that people were not getting their basic needs met. She wanted the ordinary people of Afghanistan, especially the women and children, to be able to receive healthcare.

She undertook this trek as a medical doctor … to provide treatment to people who lived in an extremely remote region who had little to no healthcare available. Her commitment was to make whatever difference she could.

She was a true hero; whilst scared she never let that prevent her from doing things she had to do. She would not want this tragedy to overshadow the ongoing plight of those still in the greatest of need.

Karen, you were an inspiration to everyone you met. You combined brains and beauty, intelligence, drive and kookiness in equal measure. You led an intensely packed and rich life: dancer, model, stunt plane walker, doctor and aid worker …

We are so proud of the work you were doing … You will be sorely missed by us, your family and friends, here in the UK, around the world and in Afghanistan. I hope that the legacy you leave is to inspire others to give love and aid rather than perpetuate hate and violence.

Badakhshan: considered safe and stunningly beautiful

While the Taliban insurgency has gradually spread across whole swaths of Afghanistan, mountainous Badakhshan is considered so safe that it is one of the country's few international tourist destinations.

Dirt poor but stunningly beautiful, every summer it attracts large numbers of foreigners, particularly expatriates living in Kabul.

They come to do two-week treks on foot and yak up the spectacular Wakhan corridor – the strange "pan handle" that sticks out from the north-eastern corner of Afghanistan towards China drawn up by colonial map makers to ensure a tiny buffer of land between Russian territory and the British Raj.

Most of Badakhshan is actually in neighbouring Tajikistan and the province has very few Pashtuns – the ethnic group that predominates in the south and east and from which the Taliban draws much of its support.

Indeed, it was the only province not to be captured by the Taliban before the hardline regime was toppled by US-led forces in late 2001.

But while insurgent forces are weak, travellers in the sparsely populated region have reported problems with criminal gangs. Although there has been a decrease in the cultivation of opium poppies in the area, there is a thriving illegal drugs economy. Poppy cultivation is nothing like the scale of the southern provinces, but Badakhshan is still home to large number of heroin-processing "laboratories".

It is also a major smuggling route for much of the heroin travelling northwards into Central Asia and then into Russia and Europe. The barely guarded border with Tajikistan is demarcated by a river that is easily crossed.

But in the southern areas of the province where the foreign aid workers were killed, there is a certain amount of overspill from far more dangerous areas of Nuristan, where intense fighting by cross-border insurgents operating from Pakistan have forced US troops to abandon some of their bases.

• This article was amended on 13 September 2010. The original stated that IAM workers were expelled at gunpoint and that Tom Little was among those expelled. These have been corrected. The sentence "Others [IAM workers] were put on trial, and faced the death penalty accused of trying to convert Muslims, though they were eventually also deported" has been deleted. It was members of a Christian charity, Shelter Now International, who were put on trial, not IAM workers.